Visual appearance refers to how things look to the visual system. Objects have physical properties, and in this regard, optical properties are usually most relevant together with size, texture, shape, etc. But what finally matters for vision is the appearance that objects have for an observer (human observers will be mainly referred to in this article; however, this is not restrictive to human beings since there are many other living species that have similar visual processing systems). The American Society for Testing and Materials (ASTM) as well as the International Commission on Illumination (CIE) define appearance as the aspect of visual experience by which objects are recognized [1, 2]. The ASTM specifies, furthermore, the definition of appearance from a psychophysical point of view: “perception in which the spectral and geometric aspects of a visual stimulus are integrated with its illuminating and viewing environment.” The physical properties have, of course, influence on the way things look, but also the neurophysiology and psychology of vision play a very important role in determining how objects and the world appear to humans. Thus it is important to understand visual appearance not merely in regard to physical aspects but as a sensation produced by the visual system when processing stimuli channeled by means of light – visible radiation. Visual appearance, understood as a visual sensation, i.e., what humans see, has certain attributes, which are also visual sensations. These include color, visual texture, gloss, and transparency. In the ASTM definition the “spectral aspects” refer, of course, to color, and the remaining aspects (including the perceived shape, size, etc. of objects) are usually considered as “geometric aspects.” From these visual sensations observers get information about the environment and build their knowledge of the external world, which allows them to interact with the environment and other individuals.
Historical Review of Studies on Visual Appearance
Under the heading of visual appearance a number of aspects are included such as color, texture, gloss, transparency, and also the shape and size of perceived objects or stimuli. The psychologist David Katz was probably the first to understand the various kinds of phenomena that always accompany the perception of color . He describes different modes of color appearance: surface color, film color, volume color, specular colors, transparent colors, perception of luster, etc. By the middle of the twentieth century, Arthur Pope realized that, in order to define a color with accuracy, i.e., giving account for different modes in which a color may appear, more than the three usual variables (hue, lightness, saturation) are needed [4, p. 28].
Brightness (or Lightness)
Lightness, hue, and saturation (attributes 1, 2, 3) are the usual variables by which color sensations are described. Size, shape, and location (attributes 4, 5, 6) are spatial categories of visual perception, which can be included in the category of spatial form. Flicker and sparkle (attributes 7, 8) involve temporal variations in the perception of light. Finally, transparency, glossiness, and luster (attributes 9, 10, 11) refer to the perception of different spatial distributions of light, aspects that will be included in the category of “cesia” to be described in the next section.
Richard Hunter proposed a classification of the geometric attributes of visual appearance . He defined six different types of gloss and developed instruments for the measurement of some of these phenomena: goniophotometers, diffuse-reflection and specular-reflection meters, glossmeters, diffuse and specular-transmission meters, etc. Before Hunter, August H. Pfund had already described an instrument for measuring gloss .
The American Society for Testing and Materials has standardized the measurement of various physical aspects related to visual appearance [8, 9, 10, 11] and published a glossary of standard terminology .
Robert Hunt has developed a hypothesis for the measurement of color appearance under different conditions of illumination and observation, taking color as the central point of his study . Sven Hesselgren observed that visual sensations, such as luster, reflection, and gloss, are not perceived as belonging to the color of an object but as something separate from color . Ralph Evans, like Pope, also realized that the three variables normally used to define color are not enough to characterize color under different modes of appearance . Evans concluded that it would be necessary to define at least five variables. In these conceptions, the phenomena of appearance have always been considered as aspects that accompany color sensations.
Other researchers have studied, either partially or globally, the aspects of visual appearance related to the different modes of distribution of light in space. Fabio Metelli tackled the problem of transparency from a phenomenological point of view, establishing a clear difference between physical and perceived transparency and developing a model that predicts the conditions under which the perception of achromatic transparency is possible . Osvaldo Da Pos, following Metelli, enlarged the model to study the perception of chromatic transparency .
John Hutchings studied the phenomenon of translucency and its importance for the visual qualities of food. In articles published in 1993 and 1995 he integrated all the aspects in a model of total appearance [17, 18]. However, this model is mainly oriented to the analysis of food  and for this reason includes also nonvisual aspects of appearance such as smell, taste (flavor), and also texture as felt in the mouth, in addition to visual texture. Paul Green-Armytage proposed a three-dimensional model to organize what he called “qualities of surfaces” [20, 21]. Subsequently he introduced the term “tincture,” borrowed from heraldry, to encompass color, texture, and cesia . Robert Sève tackled the specific problem of gloss . Michael Brill used the model of Metelli to formulate a series of rules for the perception of chromatic translucency .
In 2006, a technical committee of the CIE (Commission Internationale de l’Eclairage, International Commission on Illumination) published a report to set a framework for the measurement of visual appearance .
A Classification of Visual Appearance into Four Categories
Color can be defined as the aspect of visual appearance that results from the perception of different spectral compositions of light; it is what people are referring to when they say that some object or light stimulus is red, green, yellow, blue, etc.
Spatial forms, or shapes, are built in consciousness from the perception of borders and their integration, so that they enclose or delimitate a portion of space. Spatial forms can be one-dimensional (a straight line, a curved line, an irregular line, etc., which separate one portion of space from another), two-dimensional (a triangle, square, circle, irregular surfaces, etc.), or three-dimensional (a cube, a prism, a pyramid, a tetrahedron, a sphere, irregular volumes, etc.). They can have different shapes (as usually defined by the previous names), but they can also differ in size (length, surface, volume) and proportion (defined by the relationships among the dimensions of the shape: length, width, height).
Visual textures arise from the perception of sets of small elements that appear repeated in a surface or volume. The category of texture applies when these elements have no individual significance and are interpreted as making a whole, building a pattern that can be more or less regular or irregular. Visual textures can be distinguished and described by different variables or dimensions. These variables include density (the elements of the texture can be sparse, dense, or intermediate), predominant directionality (the elements can vary in direction or have no predominant directionality), and, of course, the shape and size of the relatively small elements whose repetition composes the texture. Visually perceived textures may be two-dimensional (without relief) or three-dimensional (with relief), the latter being also perceived by touch.
Beyond color, spatial forms, and textures, there are some other visual appearances that depend on the way in which differences in the spatial distribution of light are perceived. It is these spatial distributions of light that cause things to be seen as transparent or opaque, glossy or matte, or with intermediate appearances between these. Since there has not been a single global name or category to refer to these aspects, the neologism cesia has been coined for them.
In both color and cesia the relationship between stimulus and sensation is not fixed but depends on three main factors – illumination, object, and observer–, and is affected by other factors such as visual context, adaptation, contrast, etc.
Color and cesia are closely connected because of their relationship with light; both are different aspects of the perception of light that contribute to the visual appearance of objects. The dimension of lightness/darkness is shared by color and cesia, being the link that connects both phenomena. Color and cesia interact thereby expanding the countless number of different visual appearances that humans are able to perceive. When Ludwig Wittgenstein, in his Remarks on Color , is concerned with the different types of white, yellow and golden, gray and silver, “black” mirrors, etc. and when he says, “Opaqueness is not a property of the white color. Any more than transparency is a property of the green,” he is dealing, in reality, with this aspect that can be called cesia.
Models to Organize Different Aspects of Visual Appearance
Roberto Daniel Lozano, one of the members of the CIE technical committee 1-65, proposed a new approach to appearance characterization, with a model that classifies visual appearance according to three main categories (color, cesia, and spatiality), which in turn includes subcategories and shows the interrelations and intermediate aspects among them .
Increasing apparent intensities of illumination can be represented on lines which converge at white light glare. The coherence of a light source, which affects appearances, can also be represented on the diagram. “Hard” light, as from direct sunlight and under which objects cast clearly defined shadows, is represented on the line connecting transparent clear to white light glare, and ‘soft’ light, as from a heavily overcast sky and under which no shadows are cast, is represented on the line connecting translucent white to white light glare.
The various models described here can go some way to increasing awareness of the different aspects of appearance, and how they can be related, but no model can encompass the full experience of seeing the world in space and time.
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